i have trouble with that as well...but it seems the more i beg and the louder i get (ends up shouting PLEASE Master, this slave BEGS to cum, several times) most time he will grant me permission.
Sometimes He makes me wait still, sometimes i end up crying in frustration, and at the point that i'm ready to just lose total control, THEN He lets me cum. (but of course then i have to keep cumming until He says stop.)
It has been a long process of starting off slow and working your way to holding it in longer and longer.(remembers several times failing...and feeling so very dissappointed in myself, that i cried) The times when you do succeed,however, makes it all worth while, you know you have pleased Him and in the process yourself as well.You have shown him your submission and made him proud. When i do, and He offer those words of affection for being such a good little girl, i feel very cared for and loved. There is no easy answer i don't think, other than practice *weg*
but that is just my experience of course.